<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        div{
            width: 40vw;
            height: 20vh;
            background-color: violet;
            margin: 40px 0 0 40px;
            position: relative;
        }
        div::after{
            content: "";
            position: absolute;
            display: block;
            border-bottom: 1px solid black;
            transform: scale(0.5);
            width: 200%;
            /* left: -50% */
            left: 0;
            bottom: 0;
            transform-origin: left;
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>